Magnetic microparticles are used as a solid phase in various applications to bind biomaterial. One advantage of microparticles is the large area of the solid phase and short diffusion lengths. The size of microparticles is generally 0.05-10 .mu.m and they are available in different materials and already activated for many applications. Magnetic particles can be moved by using a magnetic field.
The separation methods of magnetic particles presently used include settling a reaction vessel in a magnetic field so that particles are accumulated into a so-called pellet at the bottom of the vessel. Thereafter, the liquid which is free from particles is decanted or removed by aspiration. However, the removal of the liquid from the vessel must be carried out very carefully so as not to remove the particles at the same time.
Publication EP-140787 (corresponding publication U.S. Pat. No. 4,649,116) proposes a method by which magnetic particles are separated from a liquid by using a magnetic rod which is pushed into the liquid. The particles are pulled off the rod by using a stronger magnet.
Publication WO-87/05536 suggests a device for separating magnetic particles which contains a rod movable in a vertical bore and provided with a magnet at the lower end thereof. The device is introduced into a liquid containing particles with the magnet in a lower position, whereby particles are collected at the end of the rod. When the magnet is allowed in an upper position, particles can be detached from the rod. In this way, particles can be collected and transferred from one liquid to another. The tip of the device is shaped like a cylinder tinner than the arm.
Publication WO-94/18565 proposes an assay method in which magnetic particles are separated from a liquid by using a rod comprising a conical tip and containing a movable magnet. In the application according to FIGS. 2, the rod comprises a concave and tapering tip part but particles are collected above this tip part, as shown in FIG. 2b.
However, the proposed separation devices and methods of magnetic particles are not very well-adapted to applications in which particles have to be transferred into very small vessels. The known devices are not very well-adapted for those applications either in which particles are collected from a large volume with respect to the means of separation.